Currently, IC tags are being considered as commercial products that serve as an entrance to the ubiquitous age. RF-ID (ultra-micro Radio Frequency identification) that have been developed to date include name tags, Suica smart cards, FeRAM cards. Many people expect significant expansion of the IC tag market in the future. However, the growth of IC tag market has yet to fulfill such expectation. This is because there are many problems, such as cost, security, confidentiality, etc., that must be resolved socially.
The RF-ID technology can also be applied to identification of documents with values such as bills, negotiable securities, etc. Counterfeiting of bills and the like has been a problem. One conceivable way of overcoming the problem is to implant IC tags into bills and the like. However, since IC tags are expensive and large-sized, this has not been achieved.
The price of IC tags can be reduced by decreasing the size of the IC tag chips. This is because decreasing the size of IC tag chips increases the number of IC tag chips that can be obtained from one wafer. To date, 0.4 mm-square IC tag chips have been developed. With these IC tag chips, 128-bit memory data inside the chips can be read out with 2.45 GHz microwaves (e.g., refer to Non-Patent Document 1).
On the other hand, a radiofrequency automatic identification (RF/AID) system applicable to identification of bills, credit cards, etc., has also been conceived in which elements other than IC tags are used. One example is a system described in Patent Document 1 that includes a plurality of resonators that resonate at a plurality of radio frequencies, the resonators being disposed and fixed on a substrate composed of paper or plastic by occupying random spatial positions. The plurality of resonators are passive solid resonators. The passive solid resonators include thin dipoles composed of an extended metal. In particular, the passive solid resonators are composed of a material, such as quartz crystal, belonging to a quartz family. For an RF target, the plurality of resonators on the substrate resonate when they are irradiated with radiofrequency electromagnetic waves. The positions of the resonators are recognized by detecting the resonance, thereby conducting identification.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 10-171951    Non-Patent Document 1: Mitsuo USAMI, Ultra-Micro IC Tag Chip “μ-chip” [Cho Kogata Musen IC Tag Chip, mu-chip], Oyo Buturi vol. 73, No. 8, 2004, pp. 1179-1183